1. Field of the Invention
The present invention relates generally to voltage regulating circuitry. More particularly, the invention relates to self-powered voltage regulating circuitry having hysteresis for controlling the operation of a DC to DC converter.
2. Description Relative to the Prior Art
There are a variety of characteristics that circuitry for regulating the operation of a DC to DC converter should have. First, the output of such circuitry should be a true digital signal. That is, the output should be in logic one state while a load voltage to be regulated is increasing toward a predetermined upper level, and in a logic zero state while the load voltage is decaying from the upper level toward a predetermined lower level. Second, switching into either the logic zero state or the logic one state should occur, respectively, at precisely the upper level and the lower level. This assures that the regulating circuitry has a well behaved hysteresis, so that the DC to DC converter accurately controls a load voltage within a prescribed range. Third, the regulating circuitry should draw no power directly from a source voltage of the converter. This not only eliminates the need for an external mode switch, it also prevents any direct coupling to the source which might inadvertently switch the regulating circuitry on or off.
There is known in the prior art a variety of voltage regulating circuits which embody some of these operating characteristics. It is believed, however, that none of these circuits possesses all characteristics.
For example, a simple voltage comparator circuit can provide a stable, well defined hysteresis. A comparator, however, requires power from a supply to operate, which forces the use of a switch if no supply power is to be dissipated while the regulating circuit is in its off state and power supply interactions are to be avoided.
Another prior art regulating circuit employs timing circuitry for controlling a load voltage. An arrangement of this type suffers from a disadvantage in that the timing circuitry has a switching point that can vary over a range of load voltage levels. Thus the hysteresis of the regulating circuitry is poorly defined, which makes it difficult to maintain the load voltage precisely within a prescribed range. Either an excessive or a deficient voltage can result in poor system performance. Furthermore, excess voltage can damage electrical components.